Audio amplifier producing high fidelity sound reproduction



.CDUEU IDA-453m AAIIAAA INVENTOR. Robert E. Oliver BY Attorney R. E- OLIVER Filed April 8, 1966 2 n Oh My mm A I IHWIIIIIIIIIIIIIIL EMZOrCQZOU KO tpzmwzww MZOF Em I h +m w 6 EN h h 3 -35 E h l AUDIO AMPLIFIER PRODUCING HIGH FIDELITY SOUND REPRODUCTION ApR'II s, 1969 A II. H M II I I I I I I I I L- United States atent O 3,437,756 AUDIO AMPLIFIER PRODUCING HIGH FIDELITY SOUND REPRODUCTION Robert E. Oliver, 143 Holly Hill Way, Los Gatos, Calif. 95030 Filed Apr. 8, 1966, Ser. No. 541,326 Int. Cl. H04m 1/00; H03f 1/00, 3/00 US. Cl. 179-1 5 Claims ABSTRACT OF THE DISCLOSURE My invention relates in general to the art of electronic amplification of Signals in the audio range. More particularly it relates to the amplification for the purpose of recording or reproducing of musical sounds with a high degree of fidelity and great presence, as this is known to those skilled in the art.

Many devices, circuits, and expedients have been employed in order that music may be recorded and reproduced so that when finally it reaches the ears of the listener it sounds exactly, or almost exactly, as though he were listening to a live orchestra, musician, or singer in the same room or area. These have been applied to records in the form of discs, as well as magnetic tape. Many efforts have also been made to improve the quality of sound reproduction including microphones directly from a live source and being transmitted to a large audience in a huge auditorium or out of doors. These have taken the form of various stereophonic devices and circuits, the use of woofers and tweeters, as well as special circuitry built into the amplifier itself. Similar efforts have been made to improve the quality of reception of broadcast signals received by sets in private homes.

Most of these are quite cumbersome and expensive and none of them have succeeded in duplicating the sound effect of the live music realistically despite the great effort which has been put forth in the so-called field of hi-fi. My circuit needs no special speakers. A basic single cone type speaker of good construction, well known to the art, is adequate. Commonly used cross-over networks in connection with mid-range speakers and tweeters also are completely unnecessary.

It is an object of my invention to produce a circuit which records and reproduces live music so that the ultimate sound effect to the human ear is more realistic than that obtained by any previous circuitry Whatever.

It is another object of my invention to produce a circuit which reproduces sound from existing recorded media such as records or tape, so that a more accurate approach to the natural sound is obtained in reproduction than that given by any other device now known.

It is another object of my invention to provide a means of reproducing sound from present recorded media of What has been up to the present considered good quality so that the reproduction is practically indistinguishable from live music to the average human ear.

It is finally an object of my invention to provide an improved means of reproducing sound with a high degree of fidelity which is simpler than those now in use and at the same time superior in performance to the same.

3,437,756 Patented Apr. 8, 1969 I have discovered that by utilizing more or less conventional electronic tubes and accessories in an unusual way I am able to achieve all of the above objectives. I have discovered further that by combining more or less conventional accessories but employing them in different orders of magnitude, contrary to what has been previously believed to be proper practice, I achieve the very unusual and unexpected results of my invention which is set forth in the specification which follows and in the drawing which forms a part of this specification.

The essential elements of my invention which present a radical departure from all previous practice and which practice was believed to be necessary in over forty-five years of radio reception and electronic sound reproduction is summed up below. Reference should now be had to the drawing.

For my first stage I use a tube having a medium high plate resistance of the order of 6,000 to 10,000 ohms and an amplification factor of 60 as used in my circuit. For my second stage I use a tube having a relatively low plate resistance of the order of magnitude of 300 to 4,000 ohms and an amplification factor of 7. In the combined grid circuits of my first tube I insert a variable non-inductive resistance 8 having a range of the order of 20 to 20,000 ohms. In the grid circuit of my second stage I insert a variable non-inductive low resistance 9 having a range of 20 to 2,000 ohms. The novelty of this part of my invention should be at once apparent to those skilled in the art, since this is contrary to what has heretofore been the practice, as more fully set forth below.

For my inductance coupling networks in the five stages of my circuit I use inductances having a resistance of from 50 to 3,000 ohms each and an inductance of approximately 30 henries each. The coupling capacitors between the first four stages have a capacitance of from A1 to 3 microfarads. The first two stages of my circuit utilizing elements on the order of magnitude which I have described constitute a very important part of my invention and I have chosen to call this part of my circuit the tone generators or tone conditioners.

In stages 3 and 4 I utilize parallel connections between the 4 sections of each pair of tubes. I have found that the net low plate resistance and high plate current flow thus resulting, fully maintains the effect produced by my tone generators described above and gives a good amplification factor of up to 30 per stage at the same time. Noninductive resistances 10 and 11 here are of range 0 to 10,000 ohms.

Stage 5 which connects to the output or speaker, comprises a push-pull circuit well known to the art in its construction and output function. This, as well as the rest of my circuit, is described more fully below and further reference should be had to the drawing.

For purposes of illustrating a preferred embodiment of my invention I have endeavored to designate my components in accordance with nomenclautre used for commercially available devices. Thus, for example, the tube numbers which I give are those which may be found in the Receiving Tube Manual published by the Radio Corporation of America, but any other tubes having similar characteristics may be employed.

Input terminals A and B are utilized independently to introduce signals from different sources. Input A is used for very low level sources such as those from dynamic or condenser type microphones, tape play-back heads, or similar sources. Input B is used independently of A for such sources as record play-back heads, broadcast through 22 receiving tuners, or any similar source having a greater magnitude of audio signal than that introduced at A. In each case the inputs at points A and B are the high or positive side of the input circuit each of which has its own common ground circuit return.

Looking now again at the input end of my circuit in which the numerals are designated as A and B, We see my first stage tube 1 having the plate resistance mentioned above, such as a 12AT7. By using a multiple section type tube such as this I am able to effect greater signal amplification and power output from this stage which is highly desirable when dealing with weak signal input such as that given by dynamic condenser type microphones and tape reproduce heads. This greater output results while maintaining the same signal to noise ratio existing with the lower power output of only one tube section. I may, of course, use for my first and second stages a simple triode having characteristics as set forth above. The second stage tube 2 of the plate resistance mentioned above may be section two of a 6DE7.

The number 2 section used in this tube has a current flow in the plate circuit great enough to provide a good wattage value to the signal amplified by it. Therefore the one section used here is sufficient. If it were necessary however, to use two or more number 2 tube sections, then I would employ two or more tubes to obtain the needed sound quality results. One novel purpose here, in the first stage, and in all stages in the amplifier preceding the final output stage, is to employ much greater plate current flow in order to give greater softness to the signal quality than would otherwise result from less plate current flow. This is one of the several factors adding up to the final superior quality of sound produced by this amplifier.

Tubes 3, 4, 5, and 6 are of the twin triode type, such as a 616, having an amplification factor of about 30 as used in my circuit and a late resistance of 7100 ohms per tube section. When connected as shown, the net plate resistance realized is about 1800 ohms but still with an amplification factor of 30 per stages 3 and 4. Tubes 7 and 8 comprise part of stage 5, being push-pull circuit mentioned above and may be of the type designated as number 7581.

Potentiometer 8 has a variable resistance of from about 20 to 20,000 ohms, while 9 has a range of from about 20 to 2,000 ohms. Potentiometers 10 and 11 both have a range of from zero to 10,000 ohms each. Of course, I may use a fixed resistance of a predetermined value within the above range if desired.

Cathode C bias resistors 12 through 16 are of a conventional type and value suited to each particular tube.

The resistor components of the coupling networks 17 are of the non-inductive type having any resistance from 50,000 to 200,000 ohms each.

The inductances or inductive components of the coupling networks 23 through 26 which form an important part of my invention have a DC resistance of from 50 to 3,000 ohms and an inductance of approximately 30 henries each. These may be similar to filter inductors type number 8-25 or -27 manufactured by the United Transformer Corporation of New York. The coupling capacitors 27 through 29 which are also important to my invention, are of the paper, plastic film, or metallized paper type and have capacitance of A to 3 microfarads. It is evident that this is of a much higher order of magnitude than used heretofore. Capacitors 30 and 31 in the push-pull circuit are likewise of the same type with a capacitance of approximately 4 microfarads each. Electrolytic capacitors 32 through 37 have a capacitance of from to microfarads each.

Input transformer 38 and output transformer 39 in 7 the push-pull circuit are of a conventional type well known to the art and suitable to the tubes used.

Inductance 40 is 30 to 100 henries each side of center tap, with low DC resistance of from 100 to 2,000 ohms each side of center tap.

Operation In operation the first stage of my tone generator is oriented to a high range of amplification while the second stage is oriented to a low range. By properly setting potentiometers 8 and 9, I am able to get separate emphasis of treble and bass tones and also proper blending of the two. This is especially advantageous under varying conditions as for example in homes, in theatres, and in outdoor installations. By proper adjustment of potentiometer 8, tube number 1 can give the right amount of audio high frequency drive needed while, at the same time, adjustment of potentiometer 9 will maintain good fullness and quality throughout low to high frequencies of tone. The first stage also supplies the high amplification that is needed to strengthen the weak signals from FM tuners, microphones, and magnetic tape playback heads, all of which produce signals of low value and require building up while at the same time preserving the tone quality by adjustment of potentiometer 9. Whenever a noticeable reduction in signal strength occurs in the foregoing adjustments, it may be readily compensated for by increasing the resistance of potentiometers 10 and 11 which are the volume controls of the final amplifier output.

When the circuit of my invention is used for recording from an original, live source by way of a good quality microphone such as RCA model 77DX, this may be accomplished directly into tape and there is no need for frequency compensation previous or subsequent to recording as has always been done in the past. When the tape is played back through my circuit there is again no need for playback frequency compensation as mentioned, and the output is so natural as to defy detection from the original by most persons.

I have indicated above the superior results I get when I utilize my invention for recording purposes and then reproducing with my own amplifier. I have also found, however, that reproduction of any and all existing recorded media is vastly improved by the use of my invention. I have found for example, that the records produced by the leading record manufacturers in the United States contain high pitched and harsh tones which are softened and made more natural when reproduced through the amplifying circuit of my invention. I have found this to be true as well, when reproducing bath AM and FM radio broadcast signals.

I have verified the unusual results obtained with my invention by quantitative tests as Well as by observation of the exceptional quality of tone produced. Typical examples of these tests are shown in Table A and Table B which follow. These show that in both my first stage and second stage tube output I obtain an unusually constant signal voltage output for any given grid 8 and/or 9' control resistance settings over a wide range of frequencics.

TABLE A.FIRST STAGETUBE 12AT7-TUBE VOLTAGE OUTPUT AT VARYING FREQUENCIES [Constant Signal Input=.003 Volt RMS Amplification Factor-=] Grid Control Resistance Settings of Resistor 8, Ohms Frequency,

Cycles 20 l 500 1,000 5,000 10,000 20,000 2 EMS Volts Out;

1 Maximum bass emphasis. Maximum treble emphasis.

TABLE B.SECOND STAGE-T UBE 6DE7, SECTION 2 T UBE VOLTAGE O U'll UT AT VARYIN U FREQUENCIES [Constant Signal Input=.003 Volt RMS Amplification Factor=71 Grid Control Resistance Settings of Resistor 9, Ohms Frequency,

Cycles 20 1 100 200 500 1,000 2,000 2 EMS Volts Out Maximum bass emphasis.

2 Maximum treble emphasis.

It is well known to those skilled in the art that in any audio amplification circuit a straight line frequency curve output is highly desirable for best quality of sound production.

As seen from the above tables, I maintain a virtually straight line curve over a wide range of audio frequencies, and this under all conditions of bass and treble emphasis in tone production as generated by either or both stage one and two.

This maintenance of a straight line frequency curve while at the same time generating all desired tone emphasis, accounts to some extent for the unusual effects and vastly superior results I obtain in sound output.

While I have shown a preferred embodiment of my invention herein, certain modifications will be evident to those skilled in the art and these may be made without departing from the scope and spirit of my invention. I therefore do not limit myself to the embodiment shown herein except as I do so in the claims which follow.

I claim:

1. An improved audio amplifier utilizing a plurality of electron tube amplification stages and comprising:

a first stage comprising a triode having a plate resistance of from 6,000 to 10,000 ohms, and a resistance inserted in the grid circuit of from to 20,000 ohms; a second stage comprising a triode having a plate resistance of from 300 to 4,000 ohms, and a resistance inserted in the grid circuit of from 20 to 2,000 ohms; a third stage and a fourth stage, each of said stages comprising a pair of twin triodes connected in parallel to give a low combined plate resistance, high amplification factor, and a resistance inserted in the combined grid circuits of from zero to 10,000 ohms;

a fifth stage comprising a push-pull circuit;

coupling networks connecting said stages;

means for introducing an input signal to said first stage a loudspeaker connected to the output of said fifth stage.

2. The amplifier of claim 1 in which the first stage has a triode with an amplification factor of approximately 60, and the second stage has an amplification factor of approximately 7.

3. The amplifier of claim 1 in which the pair of twin triodes of the third stage and the pair of twin triodes of the fourth stage have a net plate resistance of approximately 1,800 ohms per pair and an amplification factor of approximately per pair.

4. The amplifier of claim 1 in which each of the coupling networks of said stages one through four comprises an inductance having a DC resistance of from to 3,000 ohms.

5. The amplifier of claim 1 in which said first stage and said second stage each comprise tubes of the multiple section types.

References Cited UNITED STATES PATENTS 2,022,496 11/1935 Feldtkeller 303- ROY LAKE, Primary Examiner.

JAMES B. MULLINS, Assistant Examiner.

US. Cl. X.R. 

